The -200 winglet is developed by Quiet Wing Corp, which adds it as part of a flap modification kit certified in 2005 by the FAA, which mostly improves performance in general on the aircraft.
Aviation Partners Boeing (with information from their company website here) retrofits all 737 classic/NextGen winglets, which mostly involves strengthening the wing structure, adding flutter ballast, and replacing the wings’ ribs and cockpit systems before adding the winglet itself. The FAA got the STC for the Classics all the way back in 2003. It costs about USD 750.000 to install and offers some benefits, including improved climb and cruise performance and decreased emissions from the aircraft. More info can be found here and here.